1. Technical Field
The present invention relates to a serial communication circuit, an integrated circuit device, a physical quantity measuring device, an electronic apparatus, a moving object, a serial communication method, and the like.
2. Related Art
Serial communication is sometimes used for, for example, transfer of data on the inside of an electronic apparatus in order to reduce the number of wires, because it is difficult to realize synchronization when a wire is long in parallel communication.
As shown in FIG. 1 of JP-A-2007-285745, an SPI (Serial Peripheral Interface) for enabling full duplex communication is sometimes used as a system of the serial communication. For example, an integrated circuit device and a physical quantity measuring device including a serial communication circuit of an SPI system include four terminals for serial communication.
The four terminals are a first terminal configured to receive a slave selection signal with which a master (e.g., a host CPU) selects a slave (e.g., an integrated circuit device and a physical quantity measuring device), a second terminal configured to receive a clock signal for serial transfer, a third terminal configured to receive input data, and a fourth terminal configured to output response data. In the following explanation, to facilitate understanding of the explanation, the first terminal is also referred to as SS terminal (Slave Select terminal).
The master is expected to include the four terminals that have input and output directions that are in opposite directions.
However, among host CPUs functioning as masters, there is known a host CPU including only three terminals excluding the SS terminal. It is assumed that the host CPU including only the three terminals excluding the SS terminal is connected to, for example, only one slave. In this case, communication can be performed between the host CPU and the slave by fixing an SS terminal on the slave side (e.g., fixed to ‘0’, i.e., a low level).
In general, it is difficult to change the host CPU (change the host CPU including only the three terminals for the serial communication to the host CPU including the four terminals for the serial communication) because the influence on the entire system including the host CPU is large. That is, the integrated circuit device and the physical quantity measuring device on the slave side including the serial communication circuit of the SPI system are likely to be connected to both the host CPU including the three terminals for the serial communication and to the host CPU including the four terminals for the serial communication.
As described in JP-A-2007-230514, it is likely that the serial communication is also executed in electronic apparatuses mounted on mobile bodies such as an automobile, an airplane, a ship, and a train that require high safety. In such use, for example, when the host CPU including only the three terminals and the physical quantity measuring device are connected and caused to perform the serial communication in the SPI system, for example, an SS terminal of the physical quantity measuring device is set to ‘0’ on a substrate. However, if the SS terminal of the physical quantity measuring device changes to ‘1’, i.e., a high level due to the influence of noise or the like and the serial communication circuit on the slave side interprets the SS terminals as being unselected, the communication is immediately disabled. That is, the serial communication is unsuitable for a use such as an automobile having low reliability of communication and requiring high safety.
Therefore, in the past, in the use such as the automobile requiring high safety, a plurality of kinds of integrated circuit devices or physical quantity measuring devices on the slave side are prepared and selected according to the number of terminals for serial communication of the host CPU. Therefore, it is necessary to manufacture integrated circuit devices or physical quantity measuring devices in which, although the internal operations are equivalent, the numbers of terminals are different. This causes problems in costs and product management.